title: Complex structure of type VI peptidoglycan muramidase effector and a cognate immunity protein.
abstract: The type VI secretion system (T6SS) is a bacterial protein-export machine that is capable of delivering virulence effectors between Gram-negative bacteria. The T6SS of Pseudomonas aeruginosa transports two lytic enzymes, Tse1 and Tse3, to degrade cell-wall peptidoglycan in the periplasm of rival bacteria that are competing for niches via amidase and muramidase activities, respectively. Two cognate immunity proteins, Tsi1 and Tsi3, are produced by the bacterium to inactivate the two antibacterial effectors, thereby protecting its siblings from self-intoxication. Recently, Tse1-Tsi1 has been structurally characterized. Here, the structure of the Tse3-Tsi3 complex is reported at 1.9 Å resolution. The results reveal that Tse3 contains a C-terminal catalytic domain that adopts a soluble lytic transglycosylase (SLT) fold in which three calcium-binding sites were surprisingly observed close to the catalytic Glu residue. The electrostatic properties of the substrate-binding groove are also distinctive from those of known structures with a similar fold. All of these features imply that a unique catalytic mechanism is utilized by Tse3 in cleaving glycosidic bonds. Tsi3 comprises a single domain showing a β-sandwich architecture that is reminiscent of the immunoglobulin fold. Three loops of Tsi3 insert deeply into the groove of Tse3 and completely occlude its active site, which forms the structural basis of Tse3 inactivation. This work is the first crystallographic report describing the three-dimensional structure of the Tse3-Tsi3 effector-immunity pair.
authors.affiliation: National Laboratory of Biomacromolecules, Institute of Biophysics, Chinese Academy of Sciences, Beijing 100101, People's Republic of China.
authors.forename: Tianyu
authors.initials: T
authors.lastname: Wang
authors.forename: Jinjing
authors.initials: J
authors.lastname: Ding
authors.forename: Ying
authors.initials: Y
authors.lastname: Zhang
authors.forename: Da-Cheng
authors.initials: DC
authors.lastname: Wang
authors.forename: Wei
authors.initials: W
authors.lastname: Liu
mesh_terms: D000942:Antigens, Bacterial / Q000096:biosynthesis / Q000737:chemistry*; D001426:Bacterial Proteins / Q000096:biosynthesis / Q000737:chemistry*; D002118:Calcium / Q000737:chemistry; D020134:Catalytic Domain; D018360:Crystallography, X-Ray; D018698:Glutamic Acid / Q000737:chemistry; D007113:Immunity, Innate; D009113:Muramidase / Q000737:chemistry* / Q000276:immunology; D010457:Peptidoglycan / Q000737:chemistry* / Q000378:metabolism; D011550:Pseudomonas aeruginosa / Q000201:enzymology* / Q000276:immunology / Q000378:metabolism
chemical_list: D000942:Antigens, Bacterial; D001426:Bacterial Proteins; D010457:Peptidoglycan; D018698:Glutamic Acid; D009113:Muramidase; D002118:Calcium
keywords: calcium binding; effectors; immunity; interaction; muramidases; peptidoglycan